The national legal standards that govern when police officers are justified in using force against people trace their lineage to a 1984 case from Charlotte, North Carolina. In that case, a diabetic man’s erratic behavior during a trip to a convenience store for juice to bring up his low blood sugar led to a confrontation with officers that left him with injuries from head to foot.

Dethorne Graham’s subsequent lawsuit against police for his injuries led to a 1989 Supreme Court decision that has become the prism for evaluating how police use force. As soon as Ferguson, Missouri, police officer Darren Wilson shot Michael Brown on Aug. 9, the Graham v. Connor case became the foundational test for whether Wilson’s response was appropriate or criminal.

To most civilians, an 18-year-old unarmed man may not appear to pose a deadly threat. But a police officer’s perspective is different. And that is how an officer should be judged after the fact, Chief Justice William Rehnquist wrote in the 1989 opinion.

“The ‘reasonableness’ of a particular use of force must be judged from the perspective of a reasonable officer on the scene, rather than with the 20/20 vision of hindsight,” Rehnquist wrote.

The sequence of events that led to the death of Brown, a black man shot by a white officer, remains unclear. An autopsy paid for by Brown’s family concluded that he was shot six times, twice in the head. The shooting has prompted multiple investigations and triggered days of rioting reflecting long-simmering racial tensions in a town of mostly black residents and a majority white police force.

Attorney General Eric Holder said Thursday the incident had opened a national conversation about “the appropriate use of force and the need to ensure fair and equal treatment for everyone who comes into contact with the police.”

A grand jury is hearing evidence to determine whether Wilson, 28, who has policed the St. Louis suburbs for six years, should be charged in Brown’s death.

The key question will be: Would a reasonable police officer, with a similar background as Wilson, have responded the same way?

The answer is typically yes.

“Except in the most outrageous cases of police misconduct, juries tend to side with police officers and give them a lot of leeway,” said Woody Connette, the attorney who represented Graham.

The police force in St. Louis, where officers shot and killed a knife-wielding robbery suspect 10 days after the Ferguson shooting, also alludes to the Graham standards.

The jury that acquitted four Los Angeles police officers in the beating of Rodney King in the early 1990s was instructed to consider the Graham standards – the officers’ “reasonable perceptions” – as they deliberated.

Officers are to be judged by those standards – even if things look different to people who weren’t involved.

“What a police officer, what she perceives at the moment of application of force, may seem very different in the hard light of the following Monday morning,” said Ken Wallentine, a recently-retired police chief and former law professor in Utah. “And there’s the rub.”